A basic goal for these machines is very high quality in printed images, using a relatively inexpensive printer. This goal is implemented by the devices described below, but at the same time has been obstructed by certain characteristics--also described below--of these same devices.
Incremental printing nowadays is generally accomplished through digital manipulation of image data in one or another type of electronic digital microprocessor. All such manipulation, including the stages discussed below under the conventional designations of "image processing" and "printmasking", can be performed in a host computer, e. g. in software that operates an attached printer, or can be built into the printer--but most commonly is shared between the two.
For operations performed within the printer, as is now well known, the printer may contain either a general-purpose digital processor running programs called "firmware", or an application-specific integrated circuit (ASIC) manufactured to perform only specific functions of particular printers. In some cases the printer may use both a firmware subsystem and an ASIC.
Image processing--The fundamental task of all these devices is receiving data representing a desired image and developing from those data specific moment-by-moment commands to a printing mechanism. This task, for purposes of the present document, will be called "image processing".
Such processing typically includes, at the outset, some form of darkness and contrast control or adjustment. In a color printer, this preprocessing stage analogously also includes color conversions and any needed color corrections. Such preprocessing can handle both user-desired color modification and any known mismatch between an input-image color specification and the operating color space and gamut of the printer.
Next downstream from contrast, darkness and color corrections--and particularly important for images other than text--image processing also includes rendering or rendition techniques (such as dithering of error diffusion). A rendition stage has two principal functions, both directed to making spatial assignments of color ink spots to particular pixels.
First, it attempts to implement the relatively continuous or very fine tonal gradations of a photograph-like image, in terms of the relatively limited number of gradations which a typical inexpensive printer can produce. A digital file in a computer ordinarily is able to represent fine tonal gradations quite accurately, since data formats--although digital--usually allow for at least 256 distinct tonal levels between, for instance, pure white and dead black.
Second, in a color printer, rendition also attempts analogously to implement the relatively huge number of colors which a computer can invoke. Rendition must accomplish this in terms of the relatively limited number of colors which a typical inexpensive printer can produce.
Banding--An obstacle to highest-quality printing is caused by repeating failure of particular elements of the print mechanisms to mark properly--or consistently with other elements. Periodic artifacts arise from constant or repeating errors of inkjet trajectory, pen positioning and speed, and printing-medium positioning and speed.
For instance malfunction or misalignment of a particular inking nozzle or the like can leave a generally consistent white or light pixel row across every image region where that particular element (e. g. nozzle) is supposed to mark. In the case of misalignment, the same problem also produces excess inking across some nearby region where the same element should not be marking.
Image regions are not all equally affected by such defects. The magnitude of banding problems, or more generally dot-placement errors, varies with the tonal level or in other words dot density within an image.
We can define three regions of a tonal ramp, based on the amount of white space:
(1) highlights: These areas have ample white space and to the naked eye exhibit little in the way of banding or other dot-placement artifacts. Such artifacts are of course present, but hard to see--because small differences in dot position can represent only a relatively small fractional change (or none) in the large amount of white space that is seen. Furthermore, because the dots that are present are so far apart, and usually irregularly located, they fail to form a visual frame of reference within which a person can detect placement errors directly. PA1 (2) midtones: These parts of the tonal range are most sensitive to banding because they have small amounts of white space. Dot-placement errors are highly visible because small differences in dot position can have a large effect on how much white space is visible. Coalescence contributes further to the conspicuousness of banding and graininess because dots clump together. PA1 (3) saturated areas: These segments of the tonal range have almost no white space showing through. The large amount of colorant on the printing medium hides dot placement errors--with the exception of print-medium advance problems. Interactions between the colorant and the printing medium, however, can lead to flood banding and coalescence.
As a practical matter, the boundaries of these tonal-range segments depend in part upon the nature of the image being printed, as well as the exact character of the dot-placement errors produced by a particular printhead. Therefore these regions of the tonal ramp can be defined neither sharply nor generally.
For a rule of thumb, however, for purposes of placement-error visibility the midtone region has very roughly more than one single printed dot per four pixels--but, at the saturated end of the range, very roughly more than one single dot subtracted from full coverage, per four pixels. For example in a four-level (including zero) system, since the maximum number of dots in each pixel is three, the maximum inking in four pixels is 3.times.4=12 and the upper limit of the midtone region is 12-1=11 dots per four pixels.
In other words, the high-visibility range lies above approximately twenty-five percent coverage in single dots, but below approximately twenty-five percent in single dots deducted from the maximum possible inking level. Again, in practice the range defines itself in a functional way and not exactly in numerical terms.
Inking and coalescence--To achieve good tonal gradations and (for color printers) vivid colors, and to substantially fill the white space between addressable pixel locations, ample quantities of colorant must be deposited. Doing so, however, generally requires subsequent removal of the water or other base--for instance by evaporation and, for some print media, absorption--and this drying step can be unduly time consuming.
In addition, if a large amount of colorant is put down all at substantially the same time, within each section of an image, related adverse bulk-colorant effects arise. These include so-called "bleed" of one color into another (particularly noticeable at color boundaries that should be sharp), "cockle" or puckering of the printing medium, and even "blocking" or offset of colorant in one printed image onto the back of an adjacent sheet. In extreme cases such blocking can cause sticking of the two sheets together, or of one sheet to pieces of the printer apparatus.
All these conditions of course--like the banding problem discussed in the preceding subsection--defeat the objective of providing the highest practicable quality of printing in a relatively economical printer. Earlier efforts in this field, however, have attempted to address these obstacles.
Printmodes--One useful known technique for dealing with both the above-described problems is laying down in each pass of a printhead only a fraction of the total colorant required in each section of the image. Any areas left white or light in each pass tend to be filled in during one or more later passes.
These techniques, known as "printmodes", not only tend to control bleed, blocking and cockle by reducing the amount of colorant that is deposited on the page essentially at once, but also help greatly to conceal banding effects. Most preferably the several printing passes are overlapping, so that each swath of colorant tends to hide the kinds of banding due to periodic errors in printing-medium advance mechanisms.
For instance, even blank space between the edges of two inaccurately abutting swaths are usually covered by at least some colorant that is well within the boundaries of at least one other swath. Depending on the total number of passes, such blanks may be covered by as many as e. g. three other swaths--in a four-pass printmode--or even more. To put it another way, only one in four drops is missing along such a "blank" pixel row, and the nonuniformity is far less noticeable.
The specific partial-inking pattern employed in each pass is called a "printmask". The way in which these different patterns or masks add up to a single fully inked image is the "printmode".
Whereas the image-processing stage establishes spatial assignments of color spots to pixels, the printmasking stage establishes temporal assignments of color spots as among the several printing passes that have access to each pixel. Printmasking is ordinarily downstream from image processing.
Random masking--Although printmode techniques are very powerful, it has been noticed that they fail to fully eliminate the effects of the previously described underlying periodic errors, and in some cases may even contribute to certain kinds of periodic artifacts. Because of this, considerable very recent attention has been directed to randomization of the printmasking stage. Some such efforts are reflected in the previously enumerated patent documents relating to randomized masks, modes and location rules.
The improvement available through randomization, unfortunately, is limited because printmasks are effective in hiding dot-placement errors only within the dimensions of the mask. Therefore a maximal improvement requires that the printmask patterns be large in comparison with the overall image--for instance, a pattern width equal to one-third or more than one-half of the image width.
Designing printers that can store and use large printmasks, however, is difficult and expensive. Most efforts have therefore focused upon printmasks no wider than sixteen or thirty-two pixels, and such widths are typically only a very small fraction of a full image width.
Such printmasks therefore are necessarily replicated across the image--with like considerations for the vertical dimensions leading to a similar replication down the image. The result is a repeating pattern (FIG. 2) that is all too easily seen in the midtones.
The illustration was made with an eight-by-eight pixel mask used to print an area fill that has one dot in each of four pixels--i. e., four dots total. This "level four" tone is well within the midtone range extending very roughly from two to twenty-seven single dots in each four pixels.
As this example demonstrates, development of new and better printmasks is likely to be only a partial solution to banding and other repeating artifacts in the midtones. (It will later be seen that the pattern of FIG. 2 actually corresponds to using a certain variant of our invention, but one in which no improvement is produced for the level-four tone.)
Superpixels and dither cells for image processing--Pixel structures called "superpixels" (see examples at right in FIGS. 1 and 3) have previously been used in the early image-processing stages for various purposes. These have included establishment of effectively nonintegral numbers of drops per pixel (as in the Askeland et al. document mentioned earlier), production of vivid colors (as in the Perumal and Lin patent document and references cited therein), and scaling of images.
Image scaling refers to a preparatory process needed for printing a physically large image from an image-data block that is small in terms of numbers of pixels. In such cases it is necessary to somehow expand the small data block to provide the numbers of pixels needed for the desired large image.
Merely cloning pixel rows and columns would yield an image that could appear satisfactory from a distance but would look very coarse when viewed at close range. For good image quality under those circumstances some form of interpolation is needed, and superpixels have been used to provide it.
For all these purposes, generally speaking a relatively low-resolution original image data array is used to call up a particular superpixel pattern--which then provides sufficient pixel information for an image with some multiple of the original resolution. If the image is to be expanded further, it may be subjected to iteration of the same procedure; alternatively, the superpixel structure adopted initially may encompass a greater number of individual pixels. Superpixels may be either symmetrical or not.
In all of these uses of superpixels, the superpixel selection for each situation is repetitive and consistent, not random. The same is true of dither cells, popularly used in the rendition stage to produce a random appearance, even including the dither cells taught in the above-mentioned patent document of Perumal & Lin. Their cells, like random printmasks, are randomly or pseudorandomly derived, but once derived are used repetitively.
Conclusion--Repetitive patterns arising from systematic dot-placement errors, even in the presence of internally randomized printmask patterns, have continued to impede achievement of uniformly excellent inkjet printing--at high throughput--on all industrially important printing media. Awkwardness of overprinting fine detail in black is another adverse limitation. Thus important aspects of the technology used in the field of the invention remain amenable to useful refinement.